Chemotherapy, in pharmacology, refers to the use of drugs to kill or stop the growth of harmful microorganisms such as bacteria, viruses, fungi, and parasites. These medicines are called antimicrobial agents. The purpose of chemotherapy is to eliminate the infecting organism without harming the host.
What Is Selective Toxicity?
A key principle of chemotherapy is selective toxicity. This means the drug should specifically act on the microorganism while causing minimal harm to human cells. This is possible because microbes have structures and metabolic pathways that are different from human cells.
Examples of Selective Toxicity
- Penicillins attack bacterial cell walls (humans do not have cell walls).
- Sulfonamides block folic acid synthesis (humans obtain folic acid from diet).
- Aminoglycosides act on bacterial ribosomes which differ from human ribosomes.
The higher the selective toxicity, the safer and more effective the drug is.
Types of Antimicrobial Agents
Antimicrobial drugs can be classified based on their action on microorganisms.
1. Bactericidal Drugs
These drugs kill the microorganisms.
Examples: Penicillins, cephalosporins, aminoglycosides, fluoroquinolones.
2. Bacteriostatic Drugs
These drugs stop the growth of microorganisms but do not kill them directly. The body’s immune system then clears the infection.
Examples: Tetracyclines, sulfonamides, macrolides.
Whether a drug acts as bactericidal or bacteriostatic may depend on dose and site of infection.
Spectrum of Activity
The spectrum indicates the range of microorganisms the drug can act against.
1. Narrow-Spectrum Drugs
Active against a limited group of organisms.
Example: Penicillin G (mainly gram-positive bacteria).
2. Broad-Spectrum Drugs
Active against a wide range of microorganisms including gram-positive and gram-negative bacteria.
Examples: Tetracyclines, chloramphenicol, fluoroquinolones.
Why Spectrum Matters?
Broad-spectrum drugs are useful in unknown infections, but they can also disturb normal flora and lead to superinfections. Narrow-spectrum agents are preferred when the exact organism is known.
Mechanisms of Action of Antimicrobial Drugs
Different drugs act on different targets in the microorganism.
1. Inhibition of Cell Wall Synthesis
Many bacteria have a strong cell wall made of peptidoglycan. Drugs that block cell wall formation cause the bacteria to burst.
Examples: Penicillins, cephalosporins, vancomycin.
2. Inhibition of Protein Synthesis
Microorganisms use ribosomes to make proteins. Antimicrobials target bacterial ribosomes without affecting human ribosomes.
Examples: Macrolides, tetracyclines, aminoglycosides.
3. Inhibition of Nucleic Acid Synthesis
These drugs block DNA or RNA production in microorganisms.
Examples: Fluoroquinolones, rifampicin.
4. Inhibition of Metabolic Pathways
Some drugs block essential metabolic processes like folic acid synthesis.
Examples: Sulfonamides, trimethoprim.
5. Disruption of Cell Membrane Function
Some antimicrobials damage the cell membrane, causing leakage of essential components.
Examples: Polymyxins, amphotericin B.
Factors Affecting Antimicrobial Action
- Site of infection: Some drugs penetrate better into brain, bone, or lungs.
- Host immunity: Bacteriostatic drugs need good immune response.
- Drug interactions: Some drugs may block or enhance each other.
- Organ function: Kidney or liver disease affects drug dosing.
Antimicrobial Resistance
Resistance occurs when microorganisms change in ways that make the drug ineffective. This is one of the biggest challenges in medicine today.
Causes of Resistance
- Overuse or misuse of antibiotics
- Incomplete treatment courses
- Hospital-acquired infections
- Spontaneous mutations in bacteria
Types of Resistance
- Natural resistance: Certain organisms are naturally resistant to some drugs.
- Acquired resistance: Due to mutation or transfer of resistance genes.
Prevention of Resistance
- Use antibiotics only when necessary
- Use correct dose and duration
- Avoid self-medication
- Prefer narrow-spectrum drugs when possible
Detailed Notes:
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